1. Field of the Invention
The present invention relates to a ranging method for a BWA (Broadband Wireless Access) system, and more particularly to a ranging method for a mobile communication system using an OFDMA (Orthogonal Frequency Division Multiple Access) scheme.
2. Description of the Related Art
Typically, an OFDMA scheme is defined as a two-dimensional access scheme for combining a TDA (Time Division Access) scheme with a FDA (Frequency Division Access) scheme. In the case of transmitting data using the OFDMA scheme, OFDMA symbols are separately loaded on sub-carriers and transmitted over prescribed sub-channels. A communication system using the OFDMA scheme needs to periodically execute a ranging procedure to correctly establish a time offset between a transmission side, i.e., a Node B, and a reception side, i.e. a UE (User Equipment), and to adjust power between them.
The ranging procedure is classified into an initial ranging process, a bandwidth request ranging process, and a maintenance ranging process(=periodic ranging process), according to its objectives.
A RC(ranging code) for the maintenance ranging process corresponds to a periodic code periodically transmitted to the Node B over the UE. The maintenance ranging process is also called a periodic ranging process.
The objectives of the above three ranging processes have been defined in the IEEE(International Electrotechnical Commission) 802.16.
The ranging procedure needs ranging sub-channels and RCs, and the UE is assigned with a different RC according to the three objectives. However, the standard document prescribed in the IEEE 802.16 does not define a method for allowing the UE to assign the RC to a variety of ranging processes having different usages and a message thereof.
The IEEE 802.16 defines a prescribed scheme wherein a Node B transmits a UL_MAP(Uplink Map) message to a UE to inform the UE of reference information in uplink access. The UL_MAP message informs the UE of various information in the uplink, for example, a UE's scheduling period and a physical channel configuration, etc. The UE receives the UL_MAP message, and executes a ranging-related procedure based on information contained in the UL_MAP message. The UL_MAP message is transmitted to all the UEs of a cell over broadcast data of the Node B.
The UL_MAP message has the following configuration as shown in Table 1.
TABLE 1SyntaxSizeUL_MAP_Message_Format( ) {Management Message Type=3 8 bitsUplink channel ID 8 bitsUCD Count 8 bitsNumber of UL_MAP elements n16 bitsAllocation Start Time32 bitsBegin PHY Specific Section {for(i=1; i<n; i+n)UL_MAP_Information_Element {VariableConnection IDUIUCOffset}}}}
As shown in Table 1, a UL_MAP_Information_Element area serving as an IE (Information Element) area of a UL_MAP message includes a Connection ID(Identifier) area, a UIUC (Uplink Interval Usage Code) area, and an Offset area. The Connection ID area records information indicative of a transmission scheme therein. The transmission scheme is classified into a unicast scheme, a broadcast scheme, and a multicast scheme. The UIUC area records information indicative of the usage of offsets recorded in the offset area. For example, a number of 2 recorded in the UIUC area means that a starting offset for use in the initial ranging process is recorded in the offset areaa number of 3 recorded in the UIUC area means that a starting offset for use in either the bandwidth request ranging or the maintenance ranging process is recorded in the offset area. The offset area records a starting offset value for use in either the initial ranging process or the maintenance ranging process according to the information recorded in the UIUC area.
The conventional UL_MAP message configuration shown in the Table 1 classifies three ranging processes according to the above objectives, but it does not provide RC allocation by which an independent process for each of the three ranging processes becomes available. In other words, although the conventional UL_MAP message configuration generates a ranging mode by the use of PN (Pseudorandom Noise) code segmentation and also generates a RC available for the three objectives, the UE cannot recognize such information, i.e. the ranging mode and the RC. Therefore, it is necessary for the conventional UL_MAP message to execute a RC allocation for independently performing ranging processes having different objectives.
Typically, even an OFDMA communication system makes it possible to generate all of the near and non-line-of-sight conditions in the same manner as in a mobile communication system channel environment using other modulation and access schemes, and contains a partial signal blocking caused by wood which may affect signal attenuation and signal multipath. Therefore, there may occur a signal collision in an initial transmission step, irrespective of the type of ranging process used in a UE, and then a random seed for providing the same backoff value as in an initial access time is adapted for a signal re-access time.
A conventional Node B transmits to the UE a UCD (Uplink Channel Descriptor) message having information indicative of the backoff value in such a way that the UE identifies the backoff value. Such a UCD message will be described in Table 2.
TABLE 2SyntaxSizeNotesUCD-Message_Format( )Management Message Type=08 bitsUplink Channel ID8 bitsConfiguration Change Count8 bitsMini-slot size8 bitsRanging Backoff Start8 bitsRanging Backoff End8 bitsRequest Backoff Start8 bitsRequest Backoff End8 bitsTLV Encoded Information for the overallVariablechannelBegin PHY Specific Section {for (i=1; i<n ; i+n)Uplink_Burst_DescriptorVariable}}}
As shown in Table 2, the Node B transmits to the UE a UCD message having information indicative of a backoff value available for a re-access time provided after the lapse of an access failure time of the UE. In other words, the backoff value indicates a kind of standby time being a duration time between the start of UE's access failure time and the start of UE's re-access time. The Node B transmits to the UE the backoff values indicating standby time information for which the UE must wait for the next ranging process after failing to execute an initial ranging process. For example, for a number of 10 determined by the above syntaxes of the “Ranging Backoff Start” and the “Ranging Backoff End” shown in the Table 2, the UE must pass over the 210-times access executable chances (i.e., 1024-times access executable chances) and then execute the next ranging process according to the Truncated Binary Exponential Backoff Algorithm. In more detail, because the UE receives a UL_MAP message and its ranging access time corresponds to a 1025-th access time, a ranging operation can be executed at the 1025-th access time. However, as stated above, a RC is differently assigned to a UE according to the three ranging processes and is also dynamically assigned to the UE according to a cell status, such that a backoff value transmitted with the UCD message must be differently assigned to the UE according to the objectives of RCs.
In conclusion, a communication system using an OFDMA scheme classifies its ranging procedure into three kinds of ranging processes according to its objective. Although a RC can be differently assigned to the three ranging processes, the UE is unable to identify information indicative of the type of ranging process and is thereby unable to execute an independent ranging operation. The conventional communication system using the OFDMA scheme cannot execute dynamic allocation which is variable with a cell status and a UE's access characteristics because the UE cannot identify such ranging type information, thereby increasing the number of ranging access times of the UE's ranging procedure. As a result, the conventional communication system using the OFDMA scheme has a disadvantage in that it unavoidably increases the length of access delay time and reduces overall system performance.